This invention relates to apparatus for switching electric current, such as direct current (DC) electricity; and more particularly to such apparatus which has a mechanism for extinguishing arcs formed between switch contacts during separation.
DC electricity is used in a variety of applications such as battery powered systems, drives for motors and accessory circuits, in which contactors are used to make and break the flow of current to the load. Weight, reliability and high DC voltage switching and interrupting capability are important considerations in developing the contactor.
In many applications relatively large direct currents must be switched which produce arcs when the contacts of the contactor separate, thereby requiring a mechanism for extinguishing the arcs. Previous DC contactors and switches incorporated one or more arc extinguishing chambers, often referred to as "arc chutes" such as described in U.S. Pat. No. 5,866,864, to extinguish arcs that formed between the switch contacts. Arc extinguishing chambers comprise series of spaced apart electrically conductive splitter plates.
The self-magnetic field produced by current flowing through conductors in the contactor interacts with the arc creating a Lorentz force that drives the arc into the extinguishing chamber. In DC switching devices, permanent magnets on the sides of the series of splitter plates establish another magnetic field across the arc extinguishing chamber which directs the arc farther into the splitter plate arrangement. The arc then propagates from one splitter plate to another in the series and eventually spanning a number of gaps between the splitter plates whereby sufficient arc voltage is built up that the arc is extinguished.
The disadvantage of using permanent magnets is that the contactor is polarized whereby arc current flowing in only one direction produces a Lorentz force in a direction that drives the arc into the extinguishing chamber. The Lorentz force produced by arc current in the opposite direction inhibits the arc from moving farther into the second extinguishing chamber. A common contactor has a pair of stationary contacts and a moveable bridging contact with separate arc extinguishing chambers for each stationary contact. Therefore, the direction of the DC current determines which arc chamber of the two is active in a bidirectional contactor. However, it is desirable to provide arc extinction which is not dependent upon the direction of the arc current, thereby allowing both arc chambers to be simultaneously active and allowing the interruption of twice the magnitude of source voltage in a nonpolarized operating mode than that achievable in a permanent-magnet-based bidirectional contactor.
Some prior DC contactors employ an electromagnet to produce the magnetic field that drives the arc into arc horns with or without a splitter plate assembly. The DC current flowing through the contactor as the contacts separate also flows through the electromagnet. Thus with a direct current contactor, the electromagnet's magnetic field has a direction that interacts with the arc's current direction so that the Lorentz force always drives the arc into the extinguishing chamber.
However, contactors, that carry very large electric currents (e.g. 300-600 amps at 750-1500 volts) and have the electromagnet connected in-line with the main current carry path require a large (300 MCM gauge) conductor for the electromagnet coil. Some contactors connect the electromagnet to the arc runners which lead to a series of ferrous splitter plates. Thus, the electromagnet's conductors do not have to be excessively large as they carry current only during interruption of the arc. When the moveable contact separates from the stationary contact, an arc forms between the contacts. Through the self field of the current in the runner of the stationary contact, a Lorentz force is applied to the arc causing the arc to commutated to a pair of copper runners. As soon as the copper runners are electrically connected to the stationary contact, via the arc, the electromagnet, connected in series with one of the runners, provides a magnetic field transverse to the arc to drive the arc along the runners toward the splitter plate. While this method of electromagnet hook-up to the arc runners allows much smaller wire gauge for the electromagnet's conductors (typically 14-16 awg) the resulting bulky coil, due to its typical conventional round winding, still has a negative size impact on the contractor. Therefore the electromagnet adds significant volume to the size of the contactor. As with most devices, it is advantageous to minimize the size of the contactor.